This invention pertains generally to heat engines and more particularly to a heat engine utilizing a thermally responsive shape memory material.
Certain materials have been found to exhibit what is known as a "shape memory effect". Once impressed with a given shape by heat treatment, these materials tend to return abruptly to that shape when heated above a transition temperature. Below this temperature, the materials are highly ductile and can be deformed with substantially less force than that developed by the materials in returning to their annealed shapes.
One material which exhibits a shape memory effect is an alloy of Ni and Ti, known as "55 Nitinol". This alloy typically contains on the order of 55% Ni, by weight, and the balance Ti.
Heretofore, there have been some attempts to build heat engines utilizing shape memory materials. One such engine is described in U.S. Pat. No. 3,913,326, dated Oct. 21, 1975. This engine has a plurality of spoke-like elements of the memory material connected between a stationary pivot and a rim-like rotor which rotates about a fixed axis disposed eccentrically of the stationary pivot. The thermally responsive elements are heated on one side of the wheel and cooled on the other to produce a torque about the fixed axis.